Pointing devices, such as mice and trackballs, are well known peripherals for personal computers and workstations. Such pointing devices allow rapid relocation of the cursor on a display screen, and are useful in many text, database and graphical programs. Perhaps the most common form of pointing device is the electronic mouse; the second most common may well be the trackball.
With a mouse, the user controls the cursor by moving the mouse over a reference surface; the cursor moves a direction and distance proportional to the movement of the mouse. Although some electronic mice use reflectance of light over a reference pad, and others use a mechanical approach, most prior art mice use a ball which is on the underside of the mouse and rolls over the reference surface (such as a desktop) when the mouse is moved. In such a prior art device, the ball contacts a pair of shaft encoders and the rotation of the ball rotates the shaft encoders, which historically includes an encoding wheel having a plurality of slits therein. A light source, often an LED, is positioned on one side of the encoding wheel, while a photosensor, such as a phototransistor, is positioned substantially opposite the light source. Rotation of the encoding wheel therebetween causes a series of light pulses to be received by the photosensor, by which the rotational movement of the ball can be converted to a digital representation useable to move the cursor.
The optomechanical operation of a trackball is similar, although many structural differences exist. In a trackball, the device remains stationary while the user rotates the ball with the thumb, fingers or palm of the hand; one ergonomic trackball is shown in U.S. Pat. No. 5,122,654, assigned to the assignee of the present invention. As with the mouse, the ball in a conventional trackball typically engages a pair of shaft encoders having encoding wheels thereon. Associated with the encoding wheels are light sources and photosensors, which generate pulses when the movement of the ball causes rotation of the shaft encoders. One prior art trackball using this approach is shown in U.S. Pat. No. 5,008,528.
Although such a prior art approach has worked well for some time, with high quality mice and trackballs providing years of trouble-free use, the mechanical elements of such pointing devices necessarily limit the useful life of the device.
Additionally, in conventional electronic mice, a quadrature signal representative of the movement of the mouse is generated by the use of two pairs of LED's and photodetectors. However, the quality of the quadrature signal has often varied with the matching of the sensitivity of the photosensor to the light output of the LED. In many instances, this has required the expensive process of matching LED's and photodetectors prior to assembly. In addition, varying light outputs from the LED can create poor focus of light onto the sensor, and extreme sensitivity of photosensor output to the distance between the LED, the encoding wheel, and the photosensor.
There has therefore been a need for a photosensor which does not require matching to a particular LED or batch of LED's, while at the same time providing good response over varying LED-to-sensor distances.
In addition, many prior art mice involve the use of a mask in combination with an encoder wheel to properly distinguish rotation of the encoder wheel. Because such masks and encoder wheels are typically constructed of injection molded plastic, tolerances cannot be controlled to the precision of most semiconductor devices. This has led, effectively, to a mechanical upper limit imposed on the accuracy of the conventional optomechanical mouse, despite the fact that the forward path of software using such mice calls for the availability of ever-increasing resolution. There has therefore been a need for a cursor control device for which accuracy is not limited by the historical tolerances of injection molding.